The author guidelines for Composite Structures journal don't indicate what font size, line spacing, margin, or other formatting instructions the authors should follow when submitting their manuscripts. How should I format my manuscript?
You can just use the standard Times New Romans with 12 font size for instance and any reasonable line spacing "1, 1.15 or 1.5" should be alright. The most important thing is to stick to the citation/references style of the journal. All the rest will be edited anyway by the journal once your paper is accepted before publication.